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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Atomic layer deposition of epitaxial layers of anatase on strontium titanate single crystals: Morphological and photoelectrochemical characterization

Atomic layer deposition was used to grow epitaxial layers of anatase (001) TiO{sub 2} on the surface of SrTiO{sub 3} (100) crystals with a 3% lattice mismatch. The epilayers grow as anatase (001) as confirmed by x-ray diffraction. Atomic force microscope images of deposited films showed epitaxial layer-by-layer growth up to about 10 nm, whereas thicker films, of up to 32 nm, revealed the formation of 2–5 nm anatase nanocrystallites oriented in the (001) direction. The anatase epilayers were used as substrates for dye sensitization. The as received strontium titanate crystal was not sensitized with a ruthenium-based dye (N3) or a thiacyanine dye (G15); however, photocurrent from excited state electron injection from these dyes was observed when adsorbed on the anatase epilayers. These results show that highly ordered anatase surfaces can be grown on an easily obtained substrate crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Hybrid molecular beam epitaxy for the growth of stoichiometric BaSnO{sub 3}

Owing to its high room-temperature electron mobility and wide bandgap, BaSnO{sub 3} has recently become of significant interest for potential room-temperature oxide electronics. A hybrid molecular beam epitaxy (MBE) approach for the growth of high-quality BaSnO{sub 3} films is developed in this work. This approach employs hexamethylditin as a chemical precursor for tin, an effusion cell for barium, and a radio frequency plasma source for oxygen. BaSnO{sub 3} films were thus grown on SrTiO{sub 3} (001) and LaAlO{sub 3} (001) substrates. Growth conditions for stoichiometric BaSnO{sub 3} were identified. Reflection high-energy electron diffraction (RHEED) intensity oscillations, characteristic of a layer-by-layer growth mode were observed. A critical thickness of ∼1 nm for strain relaxation was determined for films grown on SrTiO{sub 3} using in situ RHEED. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy confirmed the cube-on-cube epitaxy and composition. The importance of precursor chemistry is discussed in the context of the MBE growth of BaSnO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Large-scale molecular dynamics simulations of TiN/TiN(001) epitaxial film growth

Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K are carried out using incident flux ratios N/Ti = 1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti = 1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti = 2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti = 4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i.e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti = 1 due to the formation and subsequent desorption of N{sub 2} molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti = 4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti = 2 films, which have smoother surfaces, is closer to layer-by-layer.

42 ENGINEERING↗
DOE OSTI2016

Structural and electronic properties of InN epitaxial layer grown on c-plane sapphire by chemical vapor deposition technique

Growth of InN epilayers on c-plane sapphire substrate by chemical vapor deposition technique using pure indium metal and ammonia as precursors has been systematically explored. It has been found that [0001] oriented indium nitride epitaxial layers with smooth surface morphology can be grown on c-plane sapphire substrates by optimizing the growth conditions. Bandgap of the film is observed to be Burstein–Moss shifted likely to be due to high background electron concentration. It has been found that the concentration of this unintentional doping decreases with the increase in the growth temperature and the ammonia flux. Epitaxial quality on the other hand deteriorates as the growth temperature increases. Moreover, the morphology of the deposited layer has been found to change from flat top islands to faceted mounds as the flow rate of ammonia increases. This phenomenon is expected to be related to the difference in surface termination character at low and high ammonia flow rates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Growth and coalescence control of inclined c-axis polar and semipolar GaN multilayer structures grown on Si(111), Si(112), and Si(115) by metalorganic vapor phase epitaxy

Herein, silicon substrates in alternative orientations from the commonly used Si(111) were used to enable the growth of polar and semipolar GaN-based structures by the metalorganic vapor phase epitaxy method. Specifically, Si(112) and Si(115) substrates were used for the epitaxial growth of nitride multilayer structures, while the same layer schemes were also deposited on Si(111) for comparison purposes. Multiple approaches were studied to examine the influence of the seed layers and the growth process conditions upon the final properties of the GaN/Si(11x) templates. Scanning electron microscope images were acquired to examine the topography of the deposited samples. It was observed that the substrate orientation and the process conditions allow control to produce an isolated GaN block growth or a coalesced layer growth, resulting in inclined c-axis GaN structures under various forms. The angles of the GaN c-axis inclination were determined by x-ray diffraction measurements and compared with the results obtained from the analysis of the atomic force microscope (AFM) images. The AFM image analysis method to determine the structure tilt was found to be a viable method to estimate the c-axis inclination angles of the isolated blocks and the not-fully coalesced layers. The quality of the grown samples was characterized by the photoluminescence method conducted at a wide range of temperatures from 77 to 297 K, and was correlated with the sample degree of coalescence. Using the free-excitation peak positions plotted as a function of temperature, analytical Bose-Einstein model parameters were fitted to obtain further information about the grown structures.

42 ENGINEERING↗
DOE OSTI2022

Growth of ultrathin Bi 2 Se 3 films by molecular beam epitaxy

Bi 2 Se 3 is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, Bi 2 Se 3 may exhibit quantum spin Hall states. However, growing coalesced ultrathin Bi 2 Se 3 films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of Bi 2 Se 3 films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of Bi 2 Se 3 before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Altogether, coalesced ultrathin Bi 2 Se 3 films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Study of the structure, structural transition, interface model, and magnetic moments of CrN grown on MgO(001) by molecular beam epitaxy

Structural phase transition is studied in high quality CrN thin films grown by molecular beam epitaxy on MgO(001) substrates. Cross-sectional transmission electron microscopy and x-ray diffraction reveal that the epitaxial relationship between CrN film and MgO substrate is [100] CrN /[100] MgO , [110] CrN /[110] MgO , and [001] CrN /[001] MgO . The films show tensile strain/compression at the CrN/MgO(001) interface, which relaxes gradually with the film growth. Temperature dependent x-ray diffraction measurements show a first-order structural phase transition. In addition to the experimental measurements, first-principles theoretical calculations have been carried out for finding a stable model for the CrN/MgO interface. Furthermore, these calculations determine two possible models for the interface, where a monolayer of chromium oxide is formed between the CrN and MgO layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Real structure of the ZnO epitaxial films on (0001) leucosapphire substrates coated by ultrathin gold layers

The real structure of ZnO films formed by magnetron sputtering on (0001) leucosapphire substrates coated by an ultrathin (less than 0.7 nm) Au buffer layer has been studied by high-resolution microscopy. It is shown that modification of the leucosapphire substrate surface by depositing ultrathin Au layers does not lead to the formation of Au clusters at the film–substrate interface but significantly improves the structural quality of ZnO epitaxial films. It is demonstrated that the simplicity and scalability of the technique used to modify the substrate surface in combination with a high (above 2 nm/s) film growth rate under magnetron sputtering make it possible to obtain high-quality (0001) ZnO epitaxial films with an area of 5–6 cm{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin transport in epitaxial magnetic manganite/ruthenate heterostructures with an LaMnO{sub 3} layer

Epitaxial La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/LaMnO{sub 3}/SrRuO{sub 3} (LSMO/LMO/SRO) heterostructures with an LMO layer 0–35 nm thick are grown by laser ablation on an NdGaO{sub 3} substrate at a high temperature. X-ray diffraction and transmission electron microscopy demonstrate sharp interfaces and epitaxial growth of the LSMO and SRO layers in the heterostructures at an LMO layer thickness of 0–35 nm. SQUID measurements of the magnetic moment of the heterostructures with an LMO layer and the data obtained with reflectometry of polarized neutrons show that the manganite LMO layer is a ferromagnet at a temperature below 150 K and strongly affects the magnetic moment of the heterostructures at low temperatures. The magnetoresistance of the mesostructure created from the heterostructure using lithography and ion etching decreases with increasing LMO layer thickness and weakly depends on the direction of an applied magnetic field. If the LMP layer is absent, a negative magnetoresistance is detected; it is likely to be caused by a negative magnetization of the SRO layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Epitaxial synthesis of diamond layers on a monocrystalline diamond substrate in a torch microwave plasmatron

The epitaxial growth of a diamond single-crystal film in a torch microwave discharge excited by a magnetron of a domestic microwave oven with the power of {<=}1 kW in an argon-hydrogen-methane mixture with a high concentration of methane (up to 25% with respect to hydrogen) at atmospheric pressure on a sub-strate of a synthetic diamond single crystal (HPHP) with the orientation (100) and 4 Multiplication-Sign 4 mm in size is obtained. A discharge with the torch diameter of {approx}2 mm and the concentration of the microwave power absorbed in the torch volume of >10{sup 3} W/cm{sup 3} is shown to be effective for epitaxial enlargement of a single crystal of synthetic diamond. The structure of the deposited film with the thickness up to 10 {mu}m with high-quality morphology is investigated with an optical microscope as well as using the methods of the Raman scattering and scanning electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Photoluminescence of Hg{sub 1-x}Cd{sub x}Te based heterostructures grown by molecular-beam epitaxy

Photoluminescence (PL) of Hg{sub 1-x}Cd{sub x}Te-based heterostructures grown by molecular-beam epitaxy (MBE) on GaAs and Si substrates has been studied. It is shown that a pronounced disruption of the long-range order in the crystal lattice is characteristic of structures of this kind. It is demonstrated that the observed disordering is mostly due to the nonequilibrium nature of MBE and can be partly eliminated by postgrowth thermal annealing. Low-temperature spectra of epitaxial layers and structures with wide potential wells are dominated by the recombination peak of an exciton localized in density-of-states tails; the energy of this peak is substantially lower than the energy gap. In quantum-well (QW) structures at low temperatures, the main PL peak is due to carrier recombination between QW levels and the energy of the emitted photon is strictly determined by the effective (with the QW levels taken into account) energy gap.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Dispersion of the refractive index of epitaxial Pb{sub 1-x}Eu{sub x}Te (0 {<=} x {<=} 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb{sub 1-x}Eu{sub x}Te (0 {<=}x {<=} 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm{sup -1} (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb{sub 1-x}Eu{sub x}Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Determination of the thickness and spectral dependence of the refractive index of Al{sub x}In{sub 1-x}Sb epitaxial layers from reflectance spectra

A nondestructive method for measuring the thicknesses of epitaxial layers of Al{sub x}In{sub 1-x}Sb alloys based on interference effects in reflectance spectra measured in a wide wavelength range (1-28 {mu}m) is implemented. The studied 0.9-3.3 {mu}m thick Al{sub x}In{sub 1-x}Sb layers are grown on highly lattice-mismatched GaAs substrates by molecular beam epitaxy. The found thicknesses are in good agreement with the independent data of scanning electron microscopy. The spectral dependence of the refractive index n(E) of Al{sub x}In{sub 1-x}Sb layers is measured both for the regions of transparency and fundamental absorption. The refractive index for the case of E < E{sub 0} was calculated by a double-oscillator model using a refined experimental dependence of the band gap on the composition E{sub 0}(x). The experimental data on the n(E) of Al{sub x}In{sub 1-x}Sb for energies E > E{sub 0} are found based on the interference pattern.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Low-temperature growth of silicon epitaxial layers codoped with erbium and oxygen atoms

The fabrication technology and properties of light-emitting Si structures codoped with erbium and oxygen are reported. The layers are deposited onto (100) Si by molecular beam epitaxy (MBE) using an Er-doped silicon sublimation source. The partial pressure of the oxygen-containing gases in the growth chamber of the MBE facility before layer growth is lower than 5 Multiplication-Sign 10{sup -10} Torr. The oxygen and erbium concentrations in the Si layers grown at 450 Degree-Sign C is {approx}1 Multiplication-Sign 10{sup 19} and 10{sup 18} cm{sup -3}, respectively. The silicon epitaxial layers codoped with erbium and oxygen have high crystal quality and yield effective photoluminescence and electroluminescence signals with the dominant optically active Er-1 center forming upon postgrowth annealing at a temperature of 800 Degree-Sign C.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg{sub 1-x}Cd{sub x}Te epitaxial films with x = 0.19-0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30-90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 {mu}m are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Magnetoabsorption in narrow-gap HgCdTe epitaxial layers in the terahertz range

The magnetoabsorption and photoconductivity spectra are investigated in the terahertz (THz) range at a temperature of T = 4.2 K for n-Hg{sub 1−x}Cd{sub x}Te bulk epitaxial layers of various compositions (both semiconductor and semimetallic) grown by molecular-beam epitaxy. Within the framework of the Kane 8 · 8 model, the electron and hole Landau levels are calculated. It is shown that, in contrast to the results of previous investigations, all observed resonance lines are related to transitions between the Landau levels of free carriers (the cyclotron resonance in the conduction band and the transitions between heavy-hole and electron Landau levels), which is evidence of the high purity and structural perfection of the samples. The possibility of using zero-gap Hg{sub 1−x}Cd{sub x}Te solid solutions as THz photodetectors tunable by magnetic field is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Anisotropy of the solid-state epitaxy of silicon carbide in silicon

A new method for the solid-state synthesis of epitaxial layers is developed, in which a substrate participates in the chemical reaction and the reaction product grows not on the substrate surface, as in traditional epitaxial methods, but inside the substrate. This method offers new opportunities for elastic-energy relaxation due to a mechanism operating only in anisotropic media, specifically, the attraction of point defects formed during the chemical reaction. The attracting point centers of dilatation form relatively stable objects, dilatation dipoles, which significantly reduce the total elastic energy. It is shown that, in crystals with cubic symmetry, the most favorable arrangement of dipoles is the 〈111〉 direction. The theory is tested by growing silicon carbide (SiC) films on Si (111) substrates by chemical reaction with carbon monoxide CO. High-quality single-crystal SiC-4H films with thicknesses of up to 100 nm are grown on Si (111). Ellipsometric analysis showed that the optical constants of the SiC-4H films are significantly anisotropic. This is caused not only by the lattice hexagonality but also by a small amount (about 2–6%) of carbon atoms remaining in the film due to dilatation dipoles. It is shown that the optical constants of the carbon impurity correspond to strongly anisotropic highly oriented pyrolytic graphite.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Surface photovoltage method for the quality control of silicon epitaxial layers on sapphire

The surface photovoltage method is used to study “silicon-on-sapphire” epitaxial layers with a thickness of 0.3–0.6 μm, which are used to fabricate p-channel MOS (metal—oxide-semiconductor) transistors with improved radiation hardness. It is shown that the manner in which the photoconductivity of the epitaxial layer decays after the end of a light pulse generated by a light-emitting diode (wavelength ∼400 nm) strongly depends on the density of structural defects in the bulk of the structure. This enables control over how a “silicon-on-sapphire” structure is formed to provide the manufacturing of MOS structures with optimal operating characteristics.

36 MATERIALS SCIENCE↗